Wikipedia says:

A 1999 meta-analysis of five studies comparing vegetarian and non-vegetarian mortality rates in Western countries found that in comparison with regular meat-eaters, mortality from ischemic heart disease was 34% lower in pescetarians, 34% lower in ovo-lacto vegetarians, 26% lower in vegans and 20% lower in occasional meat-eaters.[9]

Now it's possible that this correlation does not imply causation. Its also possible that the meat-eaters are eating low-quality processed meat, or are overcooking their meat.

Fish does pose worries about mercury. I have been considering going pescetarian at some point in the future, but if I can get the same benefit by using olive oil instead of sunflower oil then I won't bother.